Electrooptical dark trace picture tube



Dec. 12, 1950 G. LEVY ETAL ELECTROOPTICAL DARK TRACE PICTURE TUBE Filed 0G13. 23, 1948 EGON 2.(.0

INVENTORS @usm VE Ew WAL ER .s/ mr r l L AT RN Patented Dec. 12, 14950 ELECTROOPTICAL DARK TRACE PICTURE TUBE Gustave Levyand Walter Simbrat, East Orange, N. J., assignors to National Union Radio Corporation, Orange, N. J., a corporation of Dela- Waffe Application october 23, 194s, serialnNo. 56,086

Thisl invention relates to electro-optical re'- pi'oducing systems, and more particularly it relates to systems employing 'so-'called dark trace tubes.

l It has been known for some time that certain materials, such for example as normally transparent ionic crystals, when impinged upon by a beam of electrons or cathode rays, develop lo- Calized opacity areas. Examples of such crystals are the halides of the alkaline metals or of the alkaline earth metals, and also certain salts of silver, for example silver bromide. For convenience, the expression dark trace tube is used herein as referring to a cathode-ray tube where.,- in the image-producing screen is composed, at least in part, of such ionic crystal material or materials. Likewise, for convenience, the expression ionic crystal refers to those crystals which have the property of developing localized opacity areas in response to impinging electrons or cathode-ray beams and includes those crystals containing positive and negative ionized or ionizable components which are held together mainly by electrostatic forces.` v

. Accordingly, a, principal object of thisinvention relates to an improved image-producing and erasing arrangement for dark trace cathoderay tubes.

Another object is to provide a dark trace cathode-ray tube with a pair of electron guns, one of which produces a scanning beam of signal-modulated intensity, and the other of which produces an image-erasing beam of a higher'intensity.

Another object is to provide an improved method and arrangement for erasing images on the screens of dark trace tubes by employing a combined electric eld and electronic heating of the screen by means of a special cathode-ray beam.

A feature of the invention relates to an image tube of the dark trace" type having an imageproducing screen comprised of an extremely thin sheet of light transparent material such as glass. mica, or the like, on which the ionic crystal ina-'- terial is deposited. Cooperating with this screen are two electron guns, one of Whichlproduces the localized opacity image areas in response to received electric signals, and the other of which produces an intense cathode-ray beam for erasing the previously produced dark trace image.

Another feature relates to an. image-reproducing tube of the dark trace type, lwherein the image-producing screen consists of a light transparent thin sheet of backing material such. as

` 9 Claims. (Cl. 315-43) glass, mica or the like, and having a light trans parent coating of metal such as aluminum, and with the ionic crystal material sandwiched between the aluminum coating and the backing sheet. By means of a special high intensity cathode-ray beam, the previously produced dark trace image can be erased by the combined electronic heating of the backing sheet by the im pinging cathode-ray beam and also by the electrostatic eld created between the transparent aluminum coating and the said backing mate= rial.

Another feature relates to an image-producing tube of the dark trace type having an image screen comprised of a very thin plate or sheet of light transparent and electric insulating mated rial such as glass, mica or the like, which carries a coating or layer of ionic crystals. The said mica sheet is chosen of sufficient thinness so that when it is bombarded by a high intensity cathoderay beam, its temperature is raised to a point sucient to eiect erasure of the dark trace" image.

A further feature relates to a cathode-ray tube having a pair of electron guns, one of which produces a signal-modulated cathode-ray beam for setting-up a, dark trace image, and the other of which sets-up a high intensity cathode-ray beam to erase the previously set-up dark trace image. The image screen consists of a backing of light transparent material such as mica, glass or the lik'e,'having on its side facing both cathode-rat7` beams,a light transparent coating of metal such as aluminum. sandwiched between the alumbnum and the mica or glass plate is the layer of ionic crystals. In accordance with one feature of the invention, the aluminum coating is connected to the second anode of the cathode-ray tube gun system so that the cathode-ray beam from the erasing gun sets-up a relatively high potential gradient between the aluminum coating and the mica sheet. As a result, the dark trace image can be erased by the combined heating of the ionic crystals resulting from the high intensity bombardment of the mica plate and. by the saidhigh potential gradient between the aluminum coating and the said plate.

Another featurefthe invention relates to an image-producing and erasing screen for dark trace cathode-ray tubes comprised of a baclnng sheet or "plate of verythn light transparent material such as mica or glass, and a light tra-tlsparent coating of metal such as aluminum, with a layer of ionic crystals sandwiched between Athe aluminum -09a/ting and backing plate. The alu- 2,5sa,ss1

' The erasing action is also enhanced by reason of the fact that the electron transparent metal coating 26 is very close to the member 24, and being at high or second anode potential, there is a relatively high electrostatic potential gradient between the members 26 and 24. Since this potential gradient traverses the ionic crystal screen, it supplements the erasing action produced by the heating of the thin element 24. The potential of the coating can be chosen sov that in a sense it biases the ionic crystals so that when only the beam from gun 4 is acting, the crystals respond in opacity to the signal-modulated beam intensity. However, when the higher intensity beam from the erasing gun bombards the mica or glass sheet 24, it produces an enhanced potential gradient between the elements 24 and 2S of sufficient magnitude to eect erasure. In other words, the electron beam from the erasing gun 5 in striking the aluminum layer 26, releases secondary electrons, thus charging layer 26 positively and producing an increased positive field potential gradient between 25 and 24. Because or" the high intensity of the erasing beam, and since the sheet 2t is floating the negative electrons from the erasing beam which bombard the sheet 24, will impart to it a substantial negative voltage, thus accentuating the voltage gradient between the elements 24 and 26. In other words, a suitable voltage gradient is produced between the opposite faces of the ionic crystal layer 25 by the erasing gun to eiect erasure of the dark trace previously produced by the gun In order` to further accentuate the erasing action of the beam from gun 5, the surface of coating 26 facing the guns can be provided with any wellknown coating of secondary electron-emissive material so that the impingement of the erasing beam on the coating 26 releases suicient numbers of secondary electrons to raise the positive electrostatic potential of plate 26 sufficiently to exert the necessary erasing action on the ionic crystals.

The manner of using the dark trace tube above described, to project the images on a suitable screen, is well-known in the art. For example, a suitable steady light source 2l can be mounted on one side of the screen, and by means of a suitable optical system 28, it can be projected through the dark trace screen so as to illuminate the projection screen 29.

Since the gun 5 merely acts as an erasing gun, it is not necessary that it be focussed in a movable minute scanning spot on the ionic screen. For example, it may produce a beam of the abovementioned high intensity and of sucient crosssection to simultaneously illuminate the entire screen. In that event it is not necessary to employ the deiiector plates or elements 2l, 22, it being merely necessary to provide any suitable and Well-known beam-blanking arrangement which blanks-olf the beam from gun 5 at a predetermined high rate. For example, when the beam from gun 4 has executed one complete scanning of the ionic screen, the beam from gun 5 may be switched or gated on for a relatively short intant oi time, so as to erase the previouslyproduced dark trace image. Since such blanking or gating control arrangements of this type are well-known in the art, detailed description thereof is not necessary herein, and any wellknown electronic switch may be used for that purpose.

While in the foregoing, reference has been made to the production of opacity conditions within the crystal layer, it will be understood that the invention is not limited to a screen wherein only two conditions of the crystals are produced, namely complete light transparency and complete opacity. Thus the signal-modulated beam can produce areas of diierent degrees of opacity. Without limitation thereto, it is believed that the following is an acceptable theory of the tubes operation. When an ionic crystal such as potassium chloride located in an evacuated chamber is subjected to bombardment by a cathode-ray beam, there are set up Within the crystal free electrons which tend to travel in the direction of the potential gradient across the crystal. In other words, these free electrons within the crystals tend to move towards the transparent metal coating 26 which is at positive po-Y tential. Because of the presence of positive ions within the crystal lattice, a certain percentage of these free electrons are captured and form socailed color centers within the crystal body.

These color centers thereupon act as light ab.-v

sorbers for visible light. Since the crystal lattice is in continuous thermal agitation, these so-called color centers are again broken down into positive' ions and free electrons. These free electrons continue to move towards the positive coating 2B. The net result is the production of an opacity center which travels through the crystal lattice towards the coating 26 and when it reaches this coating, it disappears, thus restoring the crystal to its original light transmission characteristics.

Furthermore, while the invention is not limited to any particular dimensional parameters, in one tube that was found to produce the desired re'-` sults, the mica backing 2li had a thickness of .003 inch; the ionic crystal layer 25 had a thickness of 8 10-3 millimeters; the potential of the coating 26 was positive l0,000 volts; the current density at the coating 25 resulting from the beam of gun 4 was l0 to 150 micro-amperes; and the current density at the coating 26 from the beam of gun 5 Was l to 6 milliamperes.

The invention is not limited tc the use of two separate guns to effect the picture recording and the picture erasing. forI both functions, in which event the control grid Il can be biassed at the desired instant or instants to such a potential as to produce the necessary high intensity in the beam from gun 4 for erasing purposes, Furthermore, as pointed out above, since the elemental areas of the ionic crystal screen change their light absorption properties in response to the impinging cathode-ray beam, it is not necessary that the screen be viewed by light passing through it. The screen can be viewed directly as a direct viewing screen, and if desired, the light source 2l can be located on the same side of the screen as that from which the picture is to be viewed, so that the picture can be observed by reflected light. Accordingly, in the claims, the expression light transmitting is used in a generic sense to cover light waves which pass through the screen from one side to the other, light waves which are reflected from the screen, and light waves which are absorbed by the screen.

The expression "image-reproducing as employed in the claims and in the specication, is used in its generic sense to include the producing of opacity changes or light transmission or light absorption changes corresponding to electric signals representing any desired form of intelligence to be recorded or stored on the dark Thus the gun 4 can be used 

